JPH10282046A - Solid electrolyte oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid electrolyte oxygen sensor provided with a sensor element which achieves a higher stability with respect to thermal and service history of suddenly changing characteristic of electromotive force near a theoretical air/fuel ratio. SOLUTION: This solid electrolyte oxygen sensor is provided with a sensor element comprising an oxide ion transmitting solid electrolyte and a platinum electrode. In this case, at least one metal selected from elements of the II group, III group, V group, VI group and VIII group in the periodic table is added to the surface alone of the platinum electrode on the side of detecting a gas to be measured while the platinum is directly bonded to the added metal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid electrolyte oxygen sensor provided with a sensor element having improved stability to thermal and usage history of a sudden change in electromotive force near a stoichiometric air-fuel ratio.

[0002]

2. Description of the Related Art An exhaust gas purification system combining a three-way catalyst and an oxygen sensor for detecting a stoichiometric air-fuel ratio has a high purification performance and is therefore widely used for exhaust gas purification in automobiles and the like. The oxygen sensor used in the system positively utilizes the oxidation catalyst activity of the platinum electrode to detect the stoichiometric air-fuel ratio. In a three-way catalyst system, the detection accuracy of the stoichiometric air-fuel ratio of the oxygen sensor greatly affects the purification performance of the system. Therefore, it is important to improve the detection accuracy.

[0003] An electromotive force type air-fuel ratio sensor (oxygen sensor) comprising an oxide ion conductive solid electrolyte and a platinum electrode has a characteristic that the electromotive force changes suddenly at almost the stoichiometric air-fuel ratio. But,
For a chemically non-equilibrium mixed gas in which unburned components such as CO, HC, H ₂ , O ₂ , and NOx coexist, such as exhaust gas discharged from an automobile engine, the air-fuel ratio at which the electromotive force changes rapidly Is known to shift from the stoichiometric air-fuel ratio depending on the degree of non-equilibrium of the mixed gas. In order to prevent this, it is effective to use an oxygen sensor downstream of the three-way catalyst to make the working gas close to the chemical equilibrium. The above method is an effective method for reducing the degree of mismatch between the detected air-fuel ratio and the stoichiometric air-fuel ratio in the oxygen sensor and significantly improving the purification performance of the system. Also in this case, an oxygen sensor having a basic configuration including a conventional oxide ion conductive solid electrolyte and a platinum electrode is used.

[0004] The characteristic of the air-fuel ratio versus the electromotive force of the sensor element comprising the oxide ion conductive solid electrolyte and the platinum electrode is such that the electromotive force changes abruptly at the stoichiometric air-fuel ratio as described above.
Various proposals have been made in order to make the characteristic in which the electromotive force changes steep so that an arbitrary air-fuel ratio can be detected. More specifically, for example, a solid electrolyte oxygen detector in which a low-temperature operability is improved by adding a transition metal oxide having catalytic activity (for example, NiO) to a platinum electrode (Japanese Patent Laid-Open No. Sho 52-4)
No. 6889), a metal oxide (for example, SnO ₂ ,
A wide-range air-fuel ratio sensor (JP-A-59-100854, JP-A-59-142455) combining In ₂ O ₃ , NiO, Co ₃ O ₄ , and CuO, and a Ni film formed on a platinum electrode , combustion sensing element (Sho 60-115844 JP) was gently electromotive force gradient in the vicinity of the stoichiometric air-fuel ratio to platinum for promoting oxidation of CO to CO ₂ on the platinum electrode and the catalyst component An oxygen sensor having a catalyst layer (Japanese Patent Publication No. 6-90176) has been proposed. For the purpose of improving heat resistance, there has been proposed an oxygen sensor in which a platinum oxide contains a composite oxide of zirconium oxide and cerium oxide as an additional component (JP-A-5-256816). Further, an electrochemical sensor (oxygen sensor) having a measurement electrode (containing platinum and bismuth) that does not cause a catalytic reaction in the equilibrium adjustment in a gas mixture to be measured (Japanese Patent Application Laid-Open No. 8-510561) has been proposed. ing.

[0005]

Even if a conventional stoichiometric air-fuel ratio oxygen sensor comprising a solid oxide ion conductive solid electrolyte and a platinum electrode is operated downstream of a three-way catalyst, only a small amount of unburned components is purified. However, even if the accuracy is improved, the detection point does not completely match the stoichiometric air-fuel ratio. The reason for this is that the component discharged from the three-way catalyst has a low-reactive gas ratio (for example, a paraffin-based ratio in the total HC and a NOx ratio in the entire oxidizing gas (O ₂ , NOx)). This is because it is considered that it is higher than the upstream side of the catalyst. In particular, the three-way catalyst has an operating air-fuel ratio that is only slightly smaller than the stoichiometric air-fuel ratio, for example, at an air excess ratio (a value obtained by dividing the air-fuel ratio by the stoichiometric air-fuel ratio of 14.6).
Even if it moves to the lean side by 001, NOx suddenly
Therefore, if the characteristic shifts to the lean side during the use process of the sensor, the purification characteristic of the entire system may be deteriorated. For this reason, in the oxygen sensor used downstream of the three-way catalyst, the characteristics (static characteristics) of the air-fuel ratio to the electromotive force (static characteristics) are maintained with much higher accuracy than the conventional sensor even when used for a long time in a high temperature environment. Need to be

[0006] On the other hand, in the process of using the oxygen sensor, a small amount of solid matter generated when fuel or lubricating oil is burned may adhere to the surface of the oxygen sensor. Deposits on the surface of the oxygen sensor are separated by the porous ceramic layer coating the sensor electrode, so that they do not directly penetrate into the electrode and have little effect on the electromotive force characteristics of the sensor, It is known that characteristic changes rarely occur due to components in engine exhaust gas. As a preventive measure in such a case, the oxygen sensor described in Japanese Patent Publication No. 6-90176 described above is provided with a porous ceramic layer for reducing penetration of components that affect electromotive force characteristics. However, since a very small amount of infiltration into the electrode is unavoidable, it is necessary to reduce the variation in the characteristics of the electrode itself in order to prevent the variation in the oxygen sensor having high accuracy.

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems of the prior art, and as a result, have found that the stoichiometric air-fuel ratio utilizing the sudden change characteristic of the generated electromotive force using an oxide ion conductive solid electrolyte and a platinum electrode. Thermal and usage history of the electromotive force sudden change characteristic near the stoichiometric air-fuel ratio by adding a different element (a metal different from platinum) only to the surface of the platinum electrode on the measurement gas detection side of the oxygen sensor and directly bonding with platinum It has been found that the stability of the present invention can be improved, and the present invention has been made.

[0008]

According to the present invention, there is provided a solid electrolyte oxygen sensor having a sensor element comprising an oxide ion conductive solid electrolyte and a platinum electrode. At least one metal selected from Group II, Group III, Group V, Group VI and Group VIII elements of the periodic table is added only to the surface of And are directly bonded to each other. In the solid electrolyte oxygen sensor of the present invention, the group II element is barium (Ba), the group III element is cerium (Ce), the group V element is bismuth (Bi), and the group VI element is bismuth (Bi). Group VIII element is chromium (Cr) and Group VIII element is iron (Fe), cobalt (C
o) and nickel (Ni). In the solid electrolyte oxygen sensor of the present invention, it is preferable that the additional metal covers the surface of the platinum electrode.

Principle A heterogeneous additive element (metal) such as a transition metal or an alkaline earth metal is bonded to the surface of the platinum electrode on the detection side of the gas to be measured of the oxygen sensor of the present invention by heat treatment or the like. The electrode obtained in this manner has the following characteristics: Among the unburned reducing gas components contained in the vehicle exhaust gas, unburned reducing gas components such as CO, H ₂ and non-paraffin hydrocarbons are
It has high oxidation activity as before. However, the paraffinic hydrocarbon gas component is artificially controlled to a predetermined oxidation reaction activity by adding an additive to the platinum electrode. In a sensor configured using the electrodes, C
O, but generates a similar high electromotive force and a conventional oxygen sensor with respect to H _2, generates only a low electromotive force for paraffinic hydrocarbon gas, the comparison voltage determines the rich side or the lean side By setting the vicinity of the electromotive force generated by the paraffin-based hydrocarbon gas, the sensor senses the paraffin-based hydrocarbon gas as a concentration of only a certain percentage of the total concentration of the atmosphere. That is, the paraffin-based hydrocarbon gas concentration is detected not as the presence of all the paraffin-based hydrocarbon gas but as the presence of only a certain percentage in the atmosphere. Therefore, the oxygen sensor configured as described above can move the air-fuel ratio detection point to the rich side by an amount corresponding to the oxygen concentration appropriate for oxidizing the paraffin-based hydrocarbon gas that does not exist.

The total concentration of paraffinic hydrocarbon gas downstream of the three-way catalyst is generally 100 ppm in terms of carbon number.
Therefore, the shift of the air-fuel ratio detection point to the rich side has a slight width corresponding to about 200 ppm or less in terms of oxygen concentration. This air-fuel ratio width is as extremely narrow as 0.001 or less in excess air ratio (a value obtained by dividing the air-fuel ratio by the stoichiometric air-fuel ratio of 14.6) before the combustion of the engine. This indicates that the detection point of the oxygen sensor can be set near the most preferable point of the exhaust gas purification characteristics of the three-way catalyst by stably controlling the oxidation activity with respect to the paraffinic hydrocarbon gas. A conventional oxygen sensor using a platinum electrode generates a high electromotive force for paraffinic hydrocarbon gas as well as CO and H ₂ , but in a usage environment with many impurities, the electromotive force for paraffinic hydrocarbon gas is compared. It tends to decrease in a short time. However, in the oxygen sensor of the present invention,
Since the reaction activity of the platinum electrode on the detection side of the gas to be measured is controlled in advance, it exhibits a characteristic of exhibiting a low electromotive force with respect to paraffin-based hydrocarbon gas even in the early stage of use. There is an advantage that characteristic fluctuation does not occur significantly.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The oxygen sensor of the present invention differs from the platinum electrode surface (including a very shallow portion inside the electrode) by adding a different element (a different metal) to the platinum electrode surface and directly bonding to the platinum. The metal reacts and binds directly, and the electromotive force for the paraffinic hydrocarbon gas is reduced by the combined action of platinum and the added metal (for example, alloying, formation of an intermetallic compound). Therefore, in the platinum electrode on the side of the gas to be measured in the oxygen sensor of the present invention, the added metal is preferably present only near the surface of the platinum film, and when the thickness of the platinum film is small, it is near the interface between the electrode and the zirconia solid electrolyte. Undesirably, the added element reaches the sensor element until the response of the sensor decreases. For this reason, the thickness of the platinum film in the platinum electrode is desirably 0.5 to 2 μm.

The amount of different metals added is also an important factor. Unlike the present invention, when the dissimilar metal is added almost uniformly in the platinum electrode, the response of the sensor is deteriorated.In addition, when the platinum electrode is simply coated on the electrode without being combined with platinum, or when the porous electrode is coated on the platinum electrode. When the metal is supported inside the porous ceramic coating, the added metal only has the effect of an oxidation catalyst having low activity, and the effect of the sensor of the present invention cannot be obtained. In addition, if the amount of the dissimilar metal directly bonded to the platinum surface is too large, the electromotive force decreases not only for the paraffinic hydrocarbon gas but also for H ₂ and CO, which is not preferable. Therefore, the amount of addition of the dissimilar metal is appropriately selected according to the type and combination so as to obtain optimum sensor performance.

As a method of manufacturing the platinum electrode in the oxygen sensor of the present invention, sputtering, vacuum evaporation, CVD,
A conventional thin film forming technique such as plating may be used. As a method for controlling the oxidation reaction activity of the electrode with respect to the paraffinic hydrocarbon gas component, there are a method of applying a solution containing a dissimilar metal on a platinum electrode and then performing a heat treatment, and a method of forming a thin film such as sputtering on the platinum electrode. A method of performing heat treatment after deposition, a method of using a raw material containing a dissimilar metal when forming a platinum electrode, and the like are suitable. When a thin film forming technique such as sputtering or vacuum deposition is used, simultaneous sputtering and simultaneous vacuum deposition of platinum and an additional metal may be used. When the vapor pressure of the dissimilar metal or the compound containing the dissimilar metal is high at the temperature during the deposition treatment, the CVD method can be applied. The heat treatment atmosphere after the addition for binding the dissimilar metal to the platinum electrode is preferably a neutral or reducing atmosphere. The heat treatment causes the added metal to react with platinum, thereby stabilizing the oxidation reaction activity of the platinum electrode surface with respect to the paraffinic hydrocarbon gas component. The metal component added to the platinum electrode exists in the vicinity of the electrode surface at a relatively high concentration in a state directly bonded to the electrode, and therefore, even when the rich air-fuel ratio and lean air-fuel ratio at the time of using this sensor are repeated, the paraffin-based The oxidation reaction activity for the hydrocarbon gas component can be controlled in a stabilized state.

The above-mentioned additive metals include various metals such as alkali metals, alkaline earth metals and transition metals.
However, when the vapor pressure of the additive metal and its oxide is high, the additive metal is likely to evaporate when exposed to a high-temperature rich atmosphere and a lean atmosphere, and the stability of the characteristics of the sensor is poor.
Therefore, as an additive metal having stable sensor characteristics,
Those having a low vapor pressure in the state of a metal and its oxide are suitable. In addition, when an alloy or an intermetallic compound is formed by reacting with platinum, an additional metal that does not significantly lower the melting point is suitable. When an intermetallic compound having a relatively low melting point is formed, it is necessary to add a small amount so that the properties of platinum (a main component of the electrode) are not impaired.

[0015]

The present invention will be described in more detail with reference to the following examples (including comparative examples).

Example 1 A flat zirconia sintered solid electrolyte (φ17 mm, thickness 1 mm) was formed on both surfaces of a zirconia sintered body by φ6 m by sputtering.
A platinum film having a thickness of about 1 μm and a thickness of about 1 μm was formed, a lead wire was attached thereto, and heat treatment was performed at 800 ° C. for 1 hour in an air atmosphere to form an electrode. The obtained sensor element is a zirconia solid electrolyte cell having a basic configuration of platinum electrode / zirconia solid electrolyte / platinum electrode. The test sensor using this cell has excellent responsiveness in order to determine H ₂ and CO, which are the main reducing components of the unburned components in the engine exhaust gas, and to determine the rich or lean tendency of the sensor characteristics. C ₃ H ₈ was selected as the paraffinic hydrocarbon, and the electromotive force characteristics thereof were examined at 500 ° C.

Next, a large number of zirconia solid electrolyte sensors were separately manufactured in the same manner as described above, and an aqueous solution in which various nitrates or hydrates thereof were dissolved on the surface of one electrode of each of these cells was φ6 mm. 20 mg on a platinum electrode
It was dropped. After drying these, in a gas atmosphere saturated with steam (H ₂ O) at room temperature in a N ₂ -1% H ₂ mixed gas,
Heat treatment was performed at 800 ° C. for 1 hour to obtain a sensor element, and a test sensor was manufactured in the same manner as described above.

Table 1 below shows the types and concentrations of nitrates used for adding different metals to the electrodes of the manufactured solid electrolyte sensor. Table 1: Conditions for adding different metals ─────────────────────────────────── Added metal nitrate Nitrate concentration (mol / l) ─────────────────────────────────── Al Al (NO ₃ ) ₃ .9H ₂ O _{01 Ba Ba (NO 3) 2} 0.01 Ce Ce (NO 3) 3 · 6H 2 O 0.01 Cr Cr (NO 3) 3 · 9H 2 O 0.01 Fe Fe (NO 3) 3 · 9H 2 O 0.01 Co Co (NO ₃ ) ₃ .6H ₂ O 0.01 Ni Ni (NO ₃ ) ₃ .6H ₂ O 0.01 ─────────────────── Ｈ H of solid electrolyte sensor when no foreign metal is added
The electromotive force characteristic in FIG. 1 for _2, the electromotive force characteristic with respect to the C ₃ H ₈ in FIG. 2, showing the electromotive force characteristic with respect to the CO in FIG. As can be seen from these characteristics, the oxygen equivalent ratio at which the electromotive force changes rapidly (symbol λ ′; λ ′ = 1 corresponds to the stoichiometric air-fuel ratio)
Indicates different values depending on the difference in unburned components. In the results of FIGS. 1 to 3, when the electromotive force sudden change point is the value of the oxygen equivalent ratio where the electromotive force is 0.45 V, H ₂ (FIG. 1), C ₃ H ₈ (FIG. 2), CO (FIG. 3) About 1.6 to 1.
8, 0.7 to 0.8, about 1. The reason for this is that, for an unburned gas in which the platinum electrode exhibits high oxidizing activity, the transport speed of the gas increases as the molecular weight of the unburned gas is smaller than oxygen in the gas transport process until reaching the platinum electrode. That's why. In an atmosphere in which unburned gas and oxygen coexist, such as engine exhaust gas, the air-fuel ratio detection characteristics are additively synthesized based on the characteristics and concentration of each unburned gas.
On the other hand, the same applies to the case where the electrode exhibits low oxidation activity with respect to the unburned gas.

That is, if the electromotive force characteristic of each gas component is shifted to the rich side or the lean side as compared with the characteristics shown in FIGS. Or to the lean side. By measuring the electromotive force characteristics for H ₂ , C ₃ H ₈ , and CO from such correspondence, it is possible to expand the electromotive force characteristics in the engine exhaust gas (in detail about individual components) and accurately predict them. Can be. When an oxygen sensor is used on the downstream side of the exhaust gas purifying catalyst, the effect of hydrocarbons is larger than the contribution of H ₂ and CO among unburned components, so that the characteristics shown in FIGS. It is necessary to pay particular attention to C ₃ H ₈ , which is a paraffinic hydrocarbon.

Using the above method, various metals (Al, B
a, was added Ce), the sensor was heat treated C ₃ H
_The electromotive force characteristics for ₈ and CO were examined. The results with and without addition are shown in FIG. 4 (for C ₃ H ₈ ) and FIG. 5 (for CO). From FIG. 4, it can be seen that the effect of the added metal on the electromotive force characteristics with respect to C ₃ H ₈ is almost negligible for aluminum, but for barium alkaline earth metal, the electromotive force on the rich side (left side in the figure) can be greatly reduced. understood. This decrease in the electromotive force has the effect of shifting the λ detection point in the engine exhaust gas to the rich side. Further, even when cerium as a transition metal was added, a decrease in the electromotive force on the rich side was observed as shown in FIG. 4, indicating that there was an effect of shifting the λ detection point to the rich side. However, as shown in FIG. 5, when barium is added, the electromotive force on the lean side (right side in the figure) with respect to CO tends to increase, and when the addition concentration is too high, the rich side and the lean side are changed. , The amplitude of the electromotive force (the fluctuation width of the electromotive force) decreases. Therefore, in the case of barium addition, the addition concentration of 0.01 mol / l is the limit.

Next, among the transition metals, an iron group (Fe, Ni,
The influence of Co) and Cr was examined. Table 1 shows the additive concentration.
It is as follows. It was confirmed by XPS (X-ray photoelectron spectroscopy) that, when iron was added in the form of a nitrate aqueous solution and then heat-treated, the electrode surface was in a metallic state. FIG. 20 shows the result. FIG. 6 shows the results of measuring the electromotive force characteristics of this test sensor with respect to C ₃ H ₈ . FIG. 6 shows that iron group and Cr also have the effect of shifting the electromotive force characteristics to the rich side. As shown in FIGS. 7 (in the case of H ₂ ) and FIG. 8 (in the case of CO), the increase of the lean-side electromotive force and the significant decrease of the lean-side electromotive force with respect to H ₂ and CO as shown in FIGS. Therefore, there is no decrease in the electromotive force amplitude between the rich side and the lean side. In particular, with the addition of nickel and cobalt, the rich shift effect can be obtained even at a relatively low voltage of 0.45 V for judging the rich side or the lean side.

FIG. 9 shows the results of examining the electromotive force characteristics with respect to C ₃ H ₈ when the amount of iron added was changed and when iron was not added. From FIG. 9, the effect of lowering the lean-side electromotive force of C ₃ H ₈ is seen when the concentration of the additive solution is 0.0001 mol / l or more, and the effect becomes stronger as the concentration of the additive solution increases. In this addition amount range, FIG. 10 (in the case of H ₂ ),
As shown in FIG. 11 (in the case of CO), there is no increase in the lean-side electromotive force with respect to H ₂ and CO, and preferable characteristics are shown. However, if the addition concentration exceeds 0.1 mol / l, the addition amount on the electrode surface is too large, which causes a problem that the electromotive force responsiveness is reduced and the adhesion of the electrode to the protective coating is reduced. Therefore,
The addition amount is preferably 0.0001 to 0.1 mol / l, particularly,
0.001 to 0.01 mol / l is a preferable range in terms of characteristics. The amount of the latter is based on the diameter of the platinum electrode of 6 mm, and is 7 × 10 ^{−8 to} 7 × 10 ⁻⁷ mol / cm per unit area of the electrode.
^This corresponds to the addition amount of ² . Since transition metals such as iron group have a low vapor pressure in a metal state and an oxide state, there is almost no reduction in concentration due to evaporation in a sensor use atmosphere, and a sensor using a platinum electrode to which these metals are added has a temperature of 900 ° C. There is a characteristic that the characteristics change with time in a high temperature range is small.

FIG. 12 shows the test sensor of this example to which 20 mg of a 0.01 mol / l nitrate aqueous solution of iron was added, and the excess air ratio (λ) in the combustion exhaust gas of isobutane and air was 0.1%.
This is a result of examining the electromotive force characteristics with respect to C ₃ H ₈ at 500 ° C. for those subjected to heat treatment at 800 ° C. and 900 ° C. for 2 hours each at a temperature of 800 ° C. and 900 ° C. . From this, it has been found that a sensor to which iron is added in particular has an advantage that the characteristics do not fluctuate even when the rich side and the lean side are repeated in a high temperature range.

Example 2 Bismuth was selected as an additive metal, and 0.00001 mol / l
From 0.001 mol / l nitrate to the electrode,
After drying, a heat treatment was performed at 800 ° C. in a gas atmosphere saturated with water vapor (H ₂ O) at room temperature in a gas obtained by mixing 1% H ₂ in N ₂ . FIG. 13 shows the electromotive force characteristics of this test sensor with respect to C ₃ H ₈ . From FIG. 13, it can be seen that bismuth has a strong effect of significantly lowering the rich-side electromotive force for C ₃ H ₈ and causing a rich shift. It can be said that the effect of reducing the electromotive force is about two orders of magnitude stronger than that of iron or the like. Therefore, in the case of adding bismuth, the upper limit of addition is 0.0001 mol / l. This corresponds to an addition amount of 7 × 10 ⁻⁹ mol / cm ² per unit area of the electrode. FIG. 14 shows the electromotive force characteristics for CO. As is clear from FIG.
If the amount exceeds 001 mol / l, the electromotive force characteristics for CO are greatly affected, which is not preferable.

On the other hand, since bismuth has a low melting point in a metal state and an oxide state, it is easy to form a compound with platinum when repeatedly exposed to an atmosphere on a rich side and a lean side. For this reason, when bismuth is added at a high concentration, the platinum particles of the platinum electrode aggregate when the sensor is used at a high temperature, and the sensor resistance increases. However, this can be avoided if low concentrations are added. On the other hand, the vapor pressure of bismuth in a metal state and an oxide state is high, and bismuth is easily oxidized and reduced. Therefore, the characteristics fluctuate when repeatedly used at a high temperature at a rich air-fuel ratio and a lean air-fuel ratio. Therefore, there is a limit to the maximum operating temperature.

An electromotive force lowering effect similar to bismuth is also observed when lead is added to a platinum electrode. Therefore, a test sensor with bismuth addition (0.0001 mol / l solution 20 mg) and lead addition (0.0001 mol / l solution 20 mg) was manufactured, and the excess air ratio (λ) in the combustion exhaust gas of isobutane and air was set at 0. The heat treatment was performed at 800 ° C. and 900 ° C. for 2 hours, and the electromotive force characteristics for C ₃ H ₈ were examined in each heat treatment process. FIG.
FIG. 16 shows the test results of bismuth addition (0.0001 mol / l solution 20 mg), and FIG. 16 shows the addition of lead (0.0001 mol / l solution).
l solution (20 mg). From the comparison between FIG. 15 and FIG. 16, the electromotive force characteristics for C ₃ H ₈ at 500 ° C. are as follows by the rich side heating and the lean side heating at 900 ° C.
It can be seen that the variation is large in the case of the lead-added electrode test sensor, while it is relatively small in the case of the bismuth-added electrode sensor. From this result, it was found that the bismuth-added electrode has a relatively high stability at a temperature up to 900 ° C., although the maximum use temperature is limited.

For metals having a high vapor pressure such as bismuth and lead (at least 0.001 mmHg at 700 to 1000 ° C.), these metals or their oxides are heated, and the generated vapor is brought into contact with the heated sensor. By law,
Can be attached. This method has the advantage that the metal can react with platinum while being uniformly attached to the electrode surface.

In this way, a test sensor was manufactured as follows. This will be described with reference to FIG. About 1 μm thick on a flat zirconia sintered body solid electrolyte similar to that in Example 1.
A zirconia solid electrolyte cell 1 having a platinum film formed by a sputtering method is placed on a quartz container 2, and Bi
3-1 g of ₂ O ₃ (bismuth oxide) powder is placed in a quartz vessel 2 and placed in a quartz tube 4, and a zirconia solid electrolyte cell is placed in a stream of 20% O ₂ —N ₂ gas 5 (1 liter / minute). Bismuth was deposited on the platinum electrode by heating 1 and Bi ₂ O ₃ powder in an electric furnace 6 at 750 ° C. for 1 hour. Next, the quartz container 2 containing the Bi ₂ O ₃ powder 3 is removed from the quartz tube 4, and the solid electrolyte cell 1 to which bismuth is attached is placed in a 1% H ₂ -H ₂ O (room temperature saturated) -N ₂ gas stream.
Heat treatment was performed at 00 ° C. to cause bismuth and platinum to react.

The C of the test sensor manufactured by the above process
FIG. 18 shows the electromotive force characteristics for ₃ H ₈ . Next, in order to manufacture a test sensor by changing the bismuth addition concentration among the processing conditions, first, the processing in the arrangement shown in FIG.
1 hour, transfer the solid electrolyte cell to another quartz tube,
At 800 ° C. in a N ₂ stream, then 1% H ₂ —H ₂ O
(Saturated at room temperature) Heat treatment was performed at 800 ° C. for 1 hour in a —N ₂ gas stream. FIG. 18 also shows the characteristics of the test sensor obtained by this processing. From these results, it was found that by this method, the effect of adding a different side metal to the rich-side electromotive force can be obtained. Further, the effect of adding to the rich-side electromotive force was obtained by the same method using lead oxide. The addition of a different metal to the electrode can be carried out by using a vapor of a halide or a pyrolysis vapor generally used in the CVD method. In this case, the heating temperature of the additive material and the solid electrolyte cell There is an advantage that the addition concentration can be controlled by changing the temperature.

Example 3 In a plate-like zirconia sintered solid electrolyte cell similar to that of Example 1, chromium and iron films were deposited on the platinum film surface by sputtering at 100 ° and 600 °, respectively, and then 1% H ₂ -H 80 in a ₂ O (room temperature saturated) gas atmosphere
After 0 ° C., a heat treatment was performed at 600 ° C. in 2% O _{2 for} 30 minutes. This addition amount is 1.4 × 10 ⁻⁷ mol /
cm ² , corresponding to 8.5 × 10 ⁻⁷ mol / cm ² . FIG. 19 shows the electromotive force characteristics of this test sensor with respect to C ₃ H ₈ . The results show that the effect of lowering the electromotive force on the rich side with respect to C ₃ H ₈ was observed, and it was found that the electrode reaction activity could be controlled by adding a different metal in the sputtering method.

[0031]

As described above, according to the solid electrolyte oxygen sensor of the present invention, in the stoichiometric air-fuel ratio oxygen sensor utilizing the sudden change characteristic of the generated electromotive force using the oxide ion conductive solid electrolyte and the platinum electrode. Controls the gas reactivity of the electrode to paraffinic hydrocarbons in exhaust gas by adding a specific dissimilar metal only to the platinum electrode surface on the detection gas side and directly bonding the added dissimilar metal and platinum on the platinum surface Therefore, even when the sensor is used under the condition that the rich / lean air-fuel ratio is repeated at a high temperature, the change of the electromotive force sudden change characteristic in the atmosphere on the downstream side of the exhaust gas purifying catalyst is small. Therefore, the use of the solid electrolyte oxygen sensor of the present invention enables stable exhaust gas control.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electromotive force characteristic with respect to H ₂ of a solid electrolyte sensor in which a foreign metal is not added to the surface of a platinum electrode on a gas to be measured side.

FIG. 2 is a diagram showing an electromotive force characteristic with respect to C ₃ H ₈ of a solid electrolyte sensor in which a foreign metal is not added to the surface of a platinum electrode on the detection side of a gas to be measured.

FIG. 3 is a diagram showing an electromotive force characteristic with respect to CO of a solid electrolyte sensor in which no foreign metal is added to the surface of a platinum electrode on the side of a gas to be measured.

FIG. 4 shows that the surface of a platinum electrode on the side of a gas to be measured has Al and B
a, was added Ce, is a diagram showing an electromotive force characteristic with respect to C ₃ H ₈ of a solid electrolyte sensor heat-treated solid electrolyte sensor, and no additives.

FIG. 5 shows that the surface of a platinum electrode on the detection side of a gas to be measured has Al, B
It is a figure which shows the electromotive force characteristic with respect to CO of the solid electrolyte sensor which added and heat-processed Ce, and the solid electrolyte sensor which does not add.

FIG. 6 shows that Fe and N are formed on the surface of the platinum electrode on the detection side of the gas to be measured.
i, Co, added Cr, illustrates the electromotive force characteristic with respect to C ₃ H ₈ of a solid electrolyte sensor heat-treated solid electrolyte sensor, and no additives.

FIG. 7 shows that Fe and N are formed on the surface of the platinum electrode on the detection side of the gas to be measured.
i, Co, added Cr, illustrates the electromotive force characteristic with respect to the solid electrolyte sensor, and additive-free solid electrolyte of H ₂ sensor and heat treated.

FIG. 8: Fe, N on the surface of a platinum electrode on the detection side of a gas to be measured
It is a figure which shows the electromotive force characteristic with respect to CO of the solid electrolyte sensor which added i, Co, and Cr and heat-processed, and the solid electrolyte sensor which does not add.

FIG. 9 is a graph showing electromotive force characteristics with respect to C ₃ H ₈ when the amount of iron added to the surface of the platinum electrode on the measurement gas detection side is changed and when iron is not added.

FIG. 10 is a graph showing electromotive force characteristics with respect to H ₂ when the amount of iron added to the surface of the platinum electrode on the detection side of the measured gas is changed and when iron is not added.

FIG. 11 is a diagram showing the electromotive force characteristics with respect to CO when the amount of iron added to the surface of the platinum electrode on the detection target gas detection side is changed and when iron is not added.

FIG. 12 is a graph showing C ₃ when a solid electrolyte sensor obtained by adding iron to the surface of a platinum electrode on the side of a gas to be measured is heated at 800 ° C. and 900 ° C. in an atmosphere on a rich side and a lean side;
For H ₈ is a diagram showing changes in the electromotive force characteristic at 500 ° C..

FIG. 13 shows C _{3 in} the case where the amount of bismuth added to the surface of the platinum electrode on the detection side of the gas to be measured was changed and in the case where no addition was made.
Is a diagram showing an electromotive force characteristic with respect to H _8.

FIG. 14 is a graph showing CO in the case where the addition amount of bismuth to the surface of the platinum electrode on the detection side of the gas to be measured was changed and in the case where no addition was made.
FIG. 7 is a diagram showing electromotive force characteristics with respect to FIG.

FIG. 15 is a diagram showing electromotive force characteristics with respect to C ₃ H ₈ when a solid electrolyte sensor in which bismuth is added to the surface of a platinum electrode on the detection side of a gas to be measured is heated in a rich atmosphere or a lean atmosphere.

FIG. 16 is a diagram showing electromotive force characteristics with respect to C ₃ H ₈ when a solid electrolyte sensor in which lead is added to the surface of a platinum electrode on the side of a gas to be measured is heated in a rich atmosphere or a lean atmosphere.

FIG. 17 is a diagram for explaining a heat treatment method for the solid electrolyte cell.

18 is a diagram showing electromotive force characteristics with respect to C ₃ H ₈ of a solid electrolyte sensor in which bismuth is added to a platinum electrode by the heat treatment method shown in FIG. 17;

FIG. 19 is a view showing the electromotive force characteristics with respect to C ₃ H ₈ of a solid electrolyte sensor in which Cr and Fe are added to the surface of a platinum electrode on the detection side of a gas to be measured by sputtering.

FIG. 20 is a view showing an X-ray photoelectron spectroscopy spectrum of the electrode surface when iron is added to the surface of the platinum electrode on the side of the gas to be measured.

[Explanation of symbols]

1: solid electrolyte cell 2: quartz container 3: Bi ₂ O ₃ powder 4: quartz tube 5: 20% O ₂ -N ₂ gas 6: electric furnace

Claims (1)

[Claims] 1. A solid electrolyte oxygen sensor having a sensor element composed of an oxide ion conductive solid electrolyte and a platinum electrode, wherein only the surface of the platinum electrode on the gas detection side is measured.
Groups II, III, V, VI and VI of the periodic table
A solid electrolyte oxygen sensor to which at least one metal selected from Group II elements is added, and wherein platinum and the added metal are directly bonded.